1. Field of the Invention
This invention is directed to compounds that provide for sustained systemic concentrations of therapeutic or prophylactic agents following administration to animals. This invention is also directed to pharmaceutical compositions including and methods using such compounds.
2. State of the Art
Rapid clearance of drugs from the systemic circulation represents a major impediment to effective clinical use of therapeutic and/or prophylactic compounds. Although multiple factors can influence the systemic concentrations of drugs achieved following administration (including drug solubility, dissolution rate, first-pass metabolism, p-glycoprotein and related efflux mechanisms, hepatic/renal elimination, etc), rapid systemic clearance is a particularly significant reason for suboptimal systemic exposure to many compounds. Rapid systemic clearance may require that large doses of drug be administered to achieve a therapeutic or prophylatic effect. Such larger doses of the drug, however, may result in greater variability in drug exposure, more frequent occurrence of side effects, or decrease in patient compliance. Frequent drug administration may also be required to maintain systemic drug levels above a minimum effective concentration. This problem is particularly significant for drugs that must be maintained in a well defined concentration window to provide continuous therapeutic or prophylactic benefit while minimizing adverse effects (including for example, antibacterial agents, antiviral agents, anticancer agents, anticonvulsants, anticoagulants, etc.).
Conventional approaches to extend the systemic exposure of drugs with rapid clearance involve the use of formulation or device approaches that provide a slow or sustained release of drug within the intestinal lumen. These approaches are well known in the art and normally require that the drug be well absorbed from the large intestine, where such formulations are most likely to reside while releasing the drug. Drugs that are amenable to conventional sustained release approaches must be orally absorbed in the intestine and traverse this epithelial barrier by passive diffusion across the apical and basolateral membranes of the intestinal epithelial cells. The physicochemical features of a molecule that favor its passive uptake from the intestinal lumen into the systemic circulation include low molecular weight (e.g., <500 Da), adequate solubility, and a balance of hydrophobic and hydrophilic character (logP generally 1.5-4.0) (Navia and Chaturvedi, 1996).
Polar or hydrophilic compounds are typically poorly absorbed through an animal's intestine as there is a substantial energetic penalty for passage of such compounds across the lipid bilayers that constitute cellular membranes. Many nutrients that result from the digestion of ingested foodstuffs in animals, such as amino acids, di- and tripeptides, monosaccharides, nucleosides and water-soluble vitamins, are polar compounds whose uptake is essential to the viability of the animal. For these substances there exist specific mechanisms for active transport of the solute molecules across the apical membrane of the intestinal epithelia. This transport is frequently energized by co-transport of ions down a concentration gradient. Solute transporter proteins are generally single sub-unit, multi-transmembrane spanning polypeptides, and upon binding of their substrates are believed to undergo conformational changes which result in movement of the substrate(s) across the membrane.
Over the past 10-15 years, it has been found that a number of orally administered drugs are recognized as substrates by some of these transporter proteins, and that this active transport may largely account for the oral absorption of these molecules (Tsuji and Tamai, 1996). While in most instances the transporter substrate properties of these drugs were unanticipated discoveries made through retrospective analysis, it has been appreciated that, in principle, one might achieve good intestinal permeability for a drug by designing in recognition and uptake by a nutrient transport system. Drugs subject to active absorption in the small intestine are often unable to passively diffuse across epithelial cell membranes and are too large to pass through the tight junctions that exist between the intestinal cells. These drugs include many compounds structurally related to amino acids, dipeptides, sugars, nucleosides, etc. (for example, many cephalosporins, ACE inhibitors, AZT gabapentin, pregabalin, baclofen, etc.)
Rapid systemic clearance occurs in a portion of these drugs and such clearance requires that they be dosed frequently to maintain a therapeutic or prophylactic concentration in the systemic circulation (Bryans and Wustrow, 1999). Conventional sustained release approaches have not always been successfully applied to these drugs if they are not absorbed from the large intestine. Thus there is a significant need for effective sustained release versions of these drugs, particularly for the pediatric patient population, since drug must be administered during school hours, raising the issues of compliance, liability, and social acceptance.
One attractive pathway that might be exploitable for sustained delivery of drugs with rapid systemic clearance is the entero-hepatic circulation of bile acids (Swaan et al, 1996). Bile acids are hydroxylated steroids that play a key role in digestion and absorption of fat and lipophilic vitamins. After synthesis in the liver, they are secreted into bile and excreted by the gall bladder into the intestinal lumen where they emulsify and help solubilize lipophilic substances. Bile acids are conserved in the body by active uptake from the terminal ileum via the sodium-dependent transporter IBAT (or ASBT) and subsequent hepatic extraction by the transporter NTCP (or LBAT) located in the sinusoidal membrane of hepatocytes. This efficient mechanism to preserve the bile acid pool is termed the enterohepatic circulation (see FIG. 2). In man, the total bile acid pool (3-5 g) recirculates 6-10 times per day giving rise to a daily uptake of approximately 20-30 g of bile acids.
The high transport capacity of the bile acid pathway has been a key reason for interest in this system for drug delivery purposes. Several papers have postulated that chemical conjugates of bile acids with drugs could be used to provide liver site-directed delivery of a drug to bring about high therapeutic concentrations in the diseased liver with minimization of general toxic reactions elsewhere in the body; and gallbladder-site delivery systems of cholecystographic agents and cholesterol gallstone dissolution accelerators” (Ho, 1987). Several groups have explored these concepts in some detail, using the C-24 carboxylic acid, C-3, C-7, and C-12 hydroxyl groups of cholic acid (and other bile acids) as handles for chemically conjugating drugs or drug surrogates. (Kramer et al, 1992; Kim et al, 1993).
The most rigorous drug targeting studies using the bile acid transport pathway to date relate to work with bile acid conjugates of HMG-CoA reductase inhibitors (Kramer et al, 1994b; Petzinger et al, 1995; Kramer and Wess, 1995; Kramer et al, 1997b). Coupling of the HMG-CoA reductase inhibitor HR 780 via an amide linkage to the C-3 position of cholate, taurocholate and glycocholate afforded substrates for both the ileal and liver bile acid transporter proteins (FIG. 3). Upon oral dosing of rats, the cholate conjugate S 3554 led to specific inhibition of HMG-CoA reductase in the liver, and in contrast to the parent compound HR 780, gave significantly reduced inhibition of the enzyme in extra-hepatic organs. Companion studies that looked at the tissue distribution of radiolabeled drugs two hours after intravenous administration through the mesenteric vein of rats also showed dramatically lower systemic levels for the bile acid conjugate relative to the parent. Because inhibition of HMG-CoA reductase requires the presence of the free carboxylic acid moiety in HR 780 this data was taken to indicate that S 3554 served as a prodrug of HR 780, undergoing hydrolysis (and other uncharacterized metabolism) in the rat liver. Interestingly, uptake of S 3554 by liver did not appear to depend on the liver bile acid transporter NTCP (which prefers taurocholate conjugates), but may instead have involved another multispecific organic anion transport system on the sinusoidal hepatocyte membrane.
In summary, while the concept of harnessing the intestinal bile acid uptake pathway to enhance the absorption of poorly absorbed drugs is well appreciated, the existing art has merely demonstrated that bile acid-drug conjugates may be effectively trafficked to the liver and generally excreted into the bile, either unchanged or as some type of metabolite. The art gives no guidance as to how one prepares a composition that exploits the bile acid transport pathway and simultaneously provides therapeutically meaningful levels of a drug substance outside of the enterohepatic circulation. Furthermore, the art does not describe the potential use of the bile acid transport pathway to achieve a circulating reservoir of conjugated drug that is slowly released into the systemic circulation to provide sustained concentrations.